1. Field of the Invention
The present invention relates to a method of forming a thick-film wiring, and more particularly, to a method of forming on a substrate a thick-film wiring having a fine line-width and a relatively large film-thickness, e.g., at least about 10 μm.
2. Description of the Related Art
In recent years, with the sizes of electronic devices being reduced, the reduction in size of electronic components used in the electronic devices has been required. In these situations, thick-film wirings are needed because the wiring pattern is fine and the line resistances of the wiring pattern are reduced.
To satisfy the above-described requirements, as proposed in Japanese Patent No. 3039265, electroconductive paste is filled in the engraved portions of an intaglio plate made of a flexible resin. The paste is dried. Then, additional electroconductive paste is added to the engraved portions of the flexible resin intaglio plate to compensate for the reduction in volume of the electroconductive paste caused by the drying. Thereafter, the electroconductive paste is dried. The process in which additional electroconductive paste is added and, then, dried is repeated several times. An adhesive layer is formed on a substrate. The intaglio plate and the substrate are bonded to each other so that the electroconductive paste is transferred onto the substrate. The electroconductive paste is fired to produce a thick-film wiring.
However, according to this method, it is necessary to repeat the filling of the electroconductive paste and the drying several times. Thus, because the number of processing steps is large, the processing time is long.
Japanese Unexamined Patent Application Publication No. 11-154782 proposes a method of forming a thick-film wiring on a substrate. Electroconductive paste which contains metallic powder and which is a light-sensitive-hardening type of paste is filled in the grooves of a transparent intaglio plate. The electroconductive paste is irradiated with UV rays from the back side of the intaglio plate while the electroconductive paste is in contact with a substrate. Thus, the light-sensitive hardening-reaction of the paste is caused, the electroconductive paste is hardened, and the electroconductive paste is bonded to the substrate. Thereafter, the intaglio plate is separated from the substrate. Thus, the hardened electroconductive paste is transferred to the substrate. Thereafter, the electroconductive paste is fired.
According to the above-described method, it is not necessary to repeat the filling of additional electroconductive paste and the drying of the electroconductive paste. Advantageously, the number of process steps can be reduced.
However, as illustrated in FIG. 11A, when the thick-film made of the electroconductive paste has a thickness of at least 20 μm, ultraviolet rays UV cannot reach the innermost part of the electroconductive paste P. Thus, the part of the paste P which is in contact with the groove 42 of the transparent intaglio plate 41 is hardened to form a hardened part P1. The part of the electroconductive paste P which is in contact with a substrate 40 is not hardened to form an un-hardened part P2. Therefore, when the substrate 40 is separated from the intaglio plate 41, a part of the electroconductive paste P remains in the groove 42. Thus, a transfer deficiency occurs as shown in FIG. 11B.
Thus, conventional thick-film wirings are formed with much difficulty.